With the recent trend for the reduction of weight and size of electrical appliances, there has been a growing demand for lithium secondary battery having a high energy density. Further, with the expansion of the field to which the lithium secondary battery can be applied, it has been desired to further improve the battery performance.
Extensive studies have been heretofore made of secondary battery comprising metallic lithium as a negative electrode as battery which can attain a high capacity. However, the greatest technical problem that prevents the practical use of such a secondary battery was that when charge and discharge are repeated, metallic lithium grows in the form of dendrite and eventually reaches the positive electrode, causing shortcircuiting in the battery.
To cope with this problem, a non-aqueous electrolyte secondary battery comprising as a negative electrode a carbon-based material capable of absorbing/releasing lithium such as artificial graphite and natural graphite has been proposed. In such a non-aqueous electrolyte secondary battery, lithium doesn't exist in metallic state, making it possible to inhibit the formation of dendrite and improve the battery life and safety. In particular, a non-aqueous electrolyte secondary battery comprising a graphite-based carbon material such as artificial graphite and natural graphite has been noted as one meeting the demand for higher capacity.
However, in a non-aqueous electrolyte secondary battery comprising as a negative electrode any graphite-based electrode material, singly or in admixture with other negative electrode materials capable of absorbing and releasing lithium, if an electrolyte comprising as a solvent a propylene carbonate which normally is preferably used in a lithium primary battery is used, the decomposition reaction of the solvent proceeds violently on the surface of the graphite electrode, disabling the smooth absorption of lithium into the graphite electrode and smooth release of lithium from the graphite electrode.
On the other hand, ethylene carbonate undergoes little such decomposition and thus has been widely used as a main solvent of electrolyte for non-aqueous electrolyte secondary battery. However, even if ethylene carbonate is used as a main solvent, the electrolyte decomposes on the surface of the electrode during charge and discharge, raising problems of drop of charge and discharge efficiency, deterioration of cycle performance, etc.
Further, ethylene carbonate exhibits a solidifying point as high as 36.4° C. as compared with propylene carbonate and thus is not used singly but is used normally in admixture with a low viscosity solvent. For this reason, the electrolyte for lithium secondary battery comprising a graphite-based negative electrode normally comprises a mixture of ethylene carbonate and diethyl carbonate. However, since most such low viscosity solvents normally have a low boiling point, it is disadvantageous in that when added in a large amount, the solvent exhibits a lowered flashpoint, though giving good electrolyte properties. On the contrary, when added only in a small amount, it is disadvantageous in electrical conductivity and viscosity at low temperatures.
Under these circumstances, Japanese Patent Laid-Open No. 1992-87156 proposes an electrolyte comprising as a solvent a mixture of a specific compound which makes unsaturated carbon-carbon bond difficultly reactive with lithium chain-like, e.g., vinylethylene carbonate and 1,2-dimethoxyethane as a low boiling solvent in the same volume as that of the specific compound in a non-aqueous electrolyte battery comprising metallic lithium as a negative electrode.
On the other hand, γ-butyrolactone, etc., which is a cyclic ester, has a high relative dielectric constant as well as a low solidifying point and thus can be used without being mixed with a low viscosity solvent. However, the γ-butyrolactone-based electrolyte, too, is subject to progress of decomposition of γ-butyrolactone on the surface of the graphite electrode during charge, causing deterioration of battery performance.
Japanese Patent Laid-Open No. 1999-31525 proposes an electrolyte solvent comprising as a main component γ-butyrolactone and as a subsidiary component ethylene carbonate in an amount of from 15 to 35% by volume and practically comprising diethyl carbonate in an amount of not smaller than 16% by volume to inhibit the decomposition of γ-butyrolactone in a non-aqueous electrolyte secondary battery comprising a graphite-based carbon material as a negative electrode.
However, the electrolyte disclosed in the above cited patent leaves something to be desired, though showing an excellent effect in its own way.
The present invention has been worked out to solve these problems. An object of the present invention is to provide a non-aqueous electrolyte secondary battery having a high energy density comprising a negative electrode containing a carbon-based material which is subject to minimized decomposition of electrolyte to exhibit a high charge and discharge efficiency and show excellent storage properties and cycle life performance even at high temperatures.